Move d6_taxonomy_term_translation and test to content_translation

Problem/Motivation

Errors when running a migration with a test db that does not have translations and content_translation is not installed. The d6_taxonomy_term_translation migration is discovered and tries to run, when it should only be discovered when content_translation is enabled.

This is the error:

pgrading d6_taxonomy_term_translation
 [error]  Drupal\Core\Database\IntegrityConstraintViolationException: SQLSTATE[23000]: Integrity constraint violation: 1048 Column 'langcode' cannot be null: UPDATE {taxonomy_term_data} SET tid=:db_update_place3
WHERE tid = :db_condition_placeholder_0; Array
(
    [:db_update_placeholder_0] => 1
    [:db_update_placeholder_1] => catalog
    [:db_update_placeholder_2] => c96b0c82-ce2a-4a3d-b883-8feeb2801430
    [:db_update_placeholder_3] => 
    [:db_condition_placeholder_0] => 1
)
 in Drupal\Core\Database\Connection->handleQueryException() (line 683 of /opt/sites/d8/core/lib/Drupal/Core/Database/Connection.php). 

Proposed resolution

Move d6_taxonomy_term_translation and tests to content translation module.

Remaining tasks

Write a patch
commit

User interface changes

N/A

API changes

N/A

Data model changes

N/A